AA6061 is a widely accepted alloy for structural extrusions. There is extensive literature on AA6061 aluminum alloys, including U.S. Pat. Nos. 6,364,969 and 6,565,679. It is typically supplied to meet minimum properties associated with the AA6061 T6 temper:                240 MPa YS-260 MPa UTS and 8% elongation for section thickness<=6.30 mm        240 MPa YS-260 MPa UTS—10% elongation for section thickness>6.30 mm        
The alloy composition can be improved using relatively low levels of Mg and Si in order to optimise extrusion speed while still meeting these mechanical property targets. An example of this is U.S. Pat. No. 6,565,679. For thick section applications (i.e. >6.30 mm or 0.25 in.) such as anti-lock brake actuator units or heavily machined engineering parts, a higher yield strength is beneficial to improve machinability and also to allow some weight reduction. Uniformity of grain structure is also important to provide uniform machinability, and also because such parts are often anodized, and a mixed recrystallized and non-recrystallized or “fibrous” grain structure can lead to an undesirable visual appearance. For this reason, a predominantly fibrous grain structure with a thin surface recrystallized layer is preferred for such applications. Often the approach to increasing strength in 6XXX alloys is to increase additions of both magnesium and silicon to achieve the required strength levels, but this can be detrimental due to the increased flow stress and reduced melting point of the alloy.
The present invention is provided to address at least some of these problems and other problems, and to provide advantages and aspects not provided by prior alloys, processing methods, and articles. A full discussion of the features and advantages of the present invention is deferred to the following detailed description.